Observation of Orientational Ordering of Incommensurate Argon Monolayers on Graphite

The relative orientation between solid argon monolayers and the basal plane of graphite single-crystal substrates has been measured as a function of mean overlayer interatomic spacing d by low-energy electron diffraction for 32 K52 K. For 3. 82 3. 95, the measured orientation varies with d as quantitatively predicted by Novaco and McTague because of periodic static distortions of the argon monolayers by lateral argon-graphite forces. The relevance of these results to Villain's calculations is discussed.